—Disease patterns, observing how a problem spreads. Gun ownership — a precursor to gun violence — can spread "much like an infectious disease circulates," said Daniel Webster, a health policy expert and co-director of the Johns Hopkins Center for Gun Policy and Research in Baltimore.

And then he explains the metaphor...

"There's sort of a contagion phenomenon" after a shooting, where people feel they need to have a gun for protection or retaliation, he said.

Are you going to say that gun sales don't jump up after mass shootings?

Typically when people say something is "contagious" like this, they mean in the way Malcolm Gladwell describes in The Turning Point. The story he tells there is of contagious suicides on some pacific island. Essentially suicide rates amongst young boys are skyhigh because it's just a thing there. Someone does it, then someone else does it, and it becomes a culturally accepted thing. Somene doing it puts the ideas in the next boy's head and makes him think it's an acceptable choice.

That's what these shootings become. They are contagious, as the more common they are the more that will occur, and the more that occur the more common they become.